88 BEHAVIOR OF PIGEONS. 



We are driven to such preposterous extremities as the result of taking a purely casual 

 feature to start with. Incubation supplies the needed heat, but that is an incidental utility 

 that has nothing to do with the nature and origin of the instinct. It enables us to see how 

 natural selection has added some minor adjustments, but explains nothing more. For 

 the real meaning of the instinct we must look to its phyletic roots. 



If we go back to animals standing near the remote ancestors of birds, to the amphibia 

 and fishes, we find the same instinct stripped of its later disguises. Here one or both parents 

 simply remain over or near the eggs and keep watchful guard against enemies. Sometimes 

 the movements of the parent serve to keep the eggs supplied with fresh water, but aeration 

 is not the purpose for which the instinct exists. 



2. Means rest and incidental protection to offspring.- The instinct is a part of the 

 reproductive cycle of activities, and always holds the same relation in all forms that exhibit 

 it, whether high or low. It follows the production of eggs or young and means primarily, 

 as I believe, rest l with incidental protection to offspring. That meaning is always manifest, 

 no less in worms, mollusks, Crustacea, spiders, and insects than in fishes, amphibia, reptiles, 

 and birds. The instinct makes no distinction between eggs and young, and that is true 

 all along the line up to birds, which extend the same blind interest to one as to the other. 



3. Essential elements of the instinct. Every essential element in the instinct of incuba- 

 tion was present long before the bird and eggs arrived. These elements are: (1) the 

 disposition to remain with or over the eggs; (2) the disposition to resist and to drive away 

 enemies; and (3) periodicity. The birds brought all these elements along in their congenital 

 equipment and added a few minor adaptations, such 'as cutting the period of incubation 

 to the need of normal development and thus avoiding indefinite waste of time in case of 

 sterile or abortive eggs. 



(1) Disposition to remain over the eggs. The disposition to remain over the eggs is 

 certainly very old and is probably bound up with the physiological necessity for rest after 

 a series of activities tending to exhaust the whole system. If this suggestion seems far- 

 fetched, when thinking of birds, it will seem less as we go back to simpler conditions, as we 

 find them among some of the lower invertebrate forms, which are relatively very inactive 

 and predisposed to remain quiet until impelled by hunger to move. Here we find animals 

 remaining over their eggs, and thus shielding them from harm, from sheer inability or indis- 

 position to move. That is the case with certain mollusks (Crepidula), the habits and devel- 

 opment of which have been recently studied by Professor Conklin. 2 Here full protection 

 to offspring is afforded without any exertion on the part of the parent in a strictly passive 

 way that excludes even any instinctive care. In Clepsine there is a manifest unwillingness 

 to leave the eggs, showing that the disposition to remain over them is instinctive. If we 

 start with forms of similar sedentary mode of life, it is easy to see that remaining over the 

 eggs would be the most likely thing to happen, even if no instinctive regard for them existed. 

 The protection afforded would, however, be quite sufficient to insure the development of the 

 instinct, natural selection favoring those individuals which kept their position unchanged 

 long enough for the eggs to hatch. 



(2) Disposition to resist enemies. The disposition to keep intruders from the vicinity 

 of the nest I have spoken of as an element of the instinct of incubation. At first sight it 

 seems to be inseparably connected with the act of covering the egg, but there are good 

 reasons for regarding it as a distinct element of behavior. In birds this element manifests 

 itself before the eggs are laid, and even before the nest is built; and in the lower animals 

 the disposition to cover the egg is not always accompanied by an aggressive attitude. This 

 attitude is one of many forms and degrees. A mild self-defensive state, in which the 



1 This is an important point in connection with the phenomenon of "weakened germs," resulting from rapid egg- 

 laying without intervening incubations a subject dealt with in Volume II. -EDITOR. 

 * Journ. of Morph., Vol. XIII, No. 1, 1897. 



